NUI Galway Lecture 4: Receptor Families PDF

Lecture 4 Receptor families John Kelly, Ph.D., Email:[email protected] Tel: (49)3268 Pharmacology and Therapeutics NUI Galway Overview At the end of this lecture you should be able to: • List the types of drug receptors • Differentiate between the different classes of receptors • Give an exa...

Lecture 4 Receptor families John Kelly, Ph.D., Email:[email protected] Tel: (49)3268 Pharmacology and Therapeutics NUI Galway Overview At the end of this lecture you should be able to: • List the types of drug receptors • Differentiate between the different classes of receptors • Give an example of each class How does the cell respond? Receptors • A bifunctional molecule • Recognises ligands which reversibly bind to a site • The combination of ligand with receptor triggers a response • There is a diverse variety of receptors with regard to structure and function, but are divided into 4 superfamilies The 4 receptor superfamilies • Ion channels • G-protein-coupled receptors (metabotropic receptors) • Kinase-linked receptors • Steroid receptors (intracellular receptors) Ion Channels • Voltage gated • Ligand gated • Both important in neurotransmission, cardiac conduction, muscle contraction, and secretion Voltage-gated Ion Channels • Open when cell membrane is depolarised – Most NB are Na+, Ca2+ and K+ • Example: voltage gated calcium channels in the heart and blood vessels – Heart rhythm and constriction of blood vessels – Targets for anti-dysrhythmics (verapamil) and antihypertensives (amlodipine) – Calcium channel blockers • Voltage gated sodium channels blocked by local anaesthetics Ligand-gated ion channels • Also called ionotropic receptors • A ligand is a small molecule that binds to a receptor • When the ligand binds to its site on the receptor, conformational change causes channel to open and ions flow through • Example: Nicotinic receptors to which the endogenous ligand acetylcholine (ACh) binds Ligand-gated ion channels • Activated by binding of chemical ligand to a site on the channel molecule – Glutamate, acetylcholine (nicotinic), GABA, ATP • Eg nicotinic acetylcholine receptor – Nicotine binds and activates the receptor – Allows Na+ and Ca2+ into the cell – Stimulant in CNS, causes enhanced neurotransmitter release – Also causes release of adrenaline from adrenal gland; fight or flight response Nicotinic Receptor • Example: Nicotinic receptors to which the endogenous ligand acetylcholine (ACh) binds to a site on the a subunits opening the channel and allowing sodium ions (Na+) to pass through the membrane Nicotinic Receptor • • • • • Resting ion channel is closed ACh or nicotine binds Channel opens Na+ and Ca2+ flow into the cell, depolarised membrane Gate closes Ligand-gated ion channel Binding sites Outside Membrane Inside Binding sites Outside Membrane Inside Ligand binds Channel opens and ions enter cell Ligand binding sites Outside Membrane Inside ions Diazepam GABA-A receptor GABA binds Cl- Channel opens Diazepam binds, channel opens more Diazepam, a benzodiazepine receptor agonist hyperpolarisation Greater hyperpolarisation G-protein-coupled receptors • Most abundant class of receptors, also called metabotropic receptors • Structure composed of seven transmembrane helices (heptahelical), with extracellular domain usually containing the ligand-binding region • Possess intracellular regions that activate G protein signalling molecules (so called as they bind the guanine nucleotides GTP and GDP) • Activate the production of second messengers [cAMP, diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP3)] • Example: b-adrenoceptors: Adrenaline/noradrenaline is endogenous ligand (3 subtypes, b1, b2 and b3) GPCRs • Effect of activation is often phosphorylation of proteins – Acts as a switch; on/off – Activation of beta adrenoceptors in heart • Phosphorylates voltage gated calcium channels • Causes the channel to open, Ca2+ comes into cell • Contracts heart muscle • Beta blockers prevent this effect G-proteins • A family of three different a, b and g subunits (“heterotrimeric”) • G protein acts as the interface between the receptor and the enzymes and/or ion channels that are the ultimate targets of GPCRs • A binding site for GDP/GTP is on the a subunit. When this is occupied by GTP, the a subunit separates from the other subunits. It is inactive when GDP is bound G-protein coupled receptor structure G-protein coupled receptor activation: sequence of events Outside Membrane Inside Ligand binding site Outside Membrane Inside Receptor Linked to a G-protein (heterotrimeric) Ligand binding site Outside Membrane Inside b a g Receptor GDP G-protein (has affinity for GDP when not associated with receptor Effector proteins are embedded in membrane or intracellular Membrane Outside b Inside Receptor a g GDP Effector proteins Enzymes Ion channels Ligand binds Outside Membrane Inside b a g Receptor GDP Effector proteins Enzymes Ion channels G-protein association, exchange of GDP for GTP on a subunit Outside Membrane Inside b a g Receptor GTP Effector proteins Enzymes Ion channels a subunit dissociates and interacts with effector molecule Outside Membrane b Receptor g a GTP Effector proteins Enzymes Ion channels Effector molecule activates secondary messengers Outside Membrane b Receptor g a GTP Effector proteins Enzymes Ion channels Intracellular events Biological Response Return to resting state following activation of effector molecule Outside Membrane Inside b a g GDP The b2 receptor • GPCR • Adrenaline and noradrenaline are endogenous ligands • crystal structure elucidated • http://www.nature.com/nature/journ al/v477/n7366/full/nature10361.html Kinase-linked recepors • • • • Also known as receptors with enzymatic cytosolic domains All are single-membrane spanning proteins Linked to an enzyme (most commonly a kinase) Cause an effect within the cell by adding or removing phosphate groups to or from specific amino acid residues. • Many of these receptors are involved in growth and differentiation • Example: Insulin receptors (a member of the tyrosine kinases, the largest group) Tyrosine kinase Outside Membrane Inside Tyr Tyr Ligand binding Outside Membrane Inside Tyr Tyr Dimerization, phosphorylation of tyrosine residues Outside Membrane Inside P Tyr Tyr P Phosphorylation of tyrosine residues on effector proteins Outside Membrane Inside Tyr P Tyr Tyr P Tyr P Insulin as an example • Promotes uptake of glucose from blood into cells for storage (for example as glycogen in the liver) • Phosphorylates and activates enzymes that synthesize glycogen, and inactivates enzymes that break down glycogen to glucose. Nuclear receptors • Also known as intracellular receptors • Thus, ligands must be lipid soluble in order to penetrate the membrane • Inactive forms are present in cytosol (sometimes nuclear) receptors that are transcription regulatory factors • Migrate to nucleus when ligand binds • Affect DNA, altering gene transcription, and thus translation of RNA into protein • Example: Steroid receptors Nuclear (intracellular) receptors Outside IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III Inside Intracellular receptor nucleus Ligand enters cell Outside IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III I I I I I I I I I I I I I II I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I I I I I II I I I I I I I I II I I I I I I I I I I I I I I I Inside Ligand binds Outside IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III Inside ligand-receptor complex enters nucleus,dimerizes binds to DNA altering cellular protein expression Outside IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III IIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIII III Inside RNA Protein Steroid Receptor example • DNA makes RNA – Transcription • RNA makes protein – Translation • Steroids activate or inhibit transcription – eg glucocorticoids inhibit transcription of the gene for cyclo-oxygenase-2 (COX-2) • anti-inflammatory • Slow response because of time to make or destroy proteins Receptor Summary Other drug targets? • Besides receptors, drug targets include: • Ion channels • Enzymes • Transporters Summary • Receptors are divided into 4 superfamilies 1. Ion channels 2. G-protein coupled receptors 3. Kinase-linked receptors 4. Nuclear (intracellular receptors) • Each has a different mechanism, but all share the property of activation by a ligand, which can be exploited by drugs